Remove the stems from the mushrooms. With a wet paper towel or mushroom brush, clean the mushroom tops so they are free of any dirt.
When your ready to roast the mushrooms, first prepare the rice. Then once that's going, roast mushrooms on a foil lined rimmed baking sheet for 20 minutes at 400F, flipping the mushroom halfway through.
When the rice is done, mix in the mashed avocado and add salt to taste.
Cut the tomatoes in half, place in a bowl and season with salt. Set aside.